At any temperature above absolute zero, particles in a system move randomly, an effect known as thermal fluctuation. The random character of the fluctuations means they cannot be put to work in a mechanical sense (the measure of the energy unavailable for work is called entropy). 19th century physicist James Clerk Maxwell proposed a tiny intelligent "demon" that could harvest the thermal fluctuations to restore their usefulness; later work in the 20th century showed that the demon itself would have entropy, which would keep the thermodynamic books balanced.

What began as a thought experiment well over a century ago has potential interest for a variety of microscopic systems today. While the intelligent demon has fallen by the wayside, contemporary researchers are replacing it with a mindless quantum mechanical device, one that can manipulate thermal fluctuations and record its actions in some kind of memory.

Particularly, Dibyendu Mandal and Christopher Jarzynski at the University of Maryland have shown theoretically that we could make a quantum demon that would extract usable energy from thermal fluctuations, store information about its actions, and then erase the memory. This is an advance on previous Maxwell-demonic work, which never described their operations in a physical way. In principle, this demon could be used to control feedback, create nanoscale devices, and perform other functions in contexts where thermal fluctuations are significant.

For their demon, the authors postulated a quantum system with three possible configurations. Many real examples of these so-called "three-state" systems exist, though the current paper did not specify a particular one to examine. In the absence of interactions, the demon transitioned randomly between the three states, which the authors labelled A, B, and C. To operate the demon, it was connected to a thermal reservoir (a material experiencing thermal fluctuations). The demon was also coupled to a mass that can move up and down, and a stream of bits from an unspecified data source.

When the coupling between the demon and the other elements were weak or turned off, the demon is equally likely to transition from A to B and A to C. However, certain interactions could induce directional transitions. Specifically, the demon could undergo clockwise (A to B to C to A) or counterclockwise (A to C to B to A) transitions, which are highly unlikely to occur in a random way. As the authors point out, this is why a three-state system is necessary: there must be at least two possible transitions to exhibit directional behavior.

The data stream consisted of bit values 0 or 1, so the state of the whole system could be described by the combination of the demon state and the bit value: A0, A1, B0, etc. Without connecting the mass and thermal reservoir, spontaneous transitions from A to B and back, or B to C and back, occurred without any restrictions.

However, flips between bit states could alter the demon's state, and transitions between demon states could change the bits. When the system was in state C0 and the next bit in line was a 1, then the system was forced into A0 next; if the system began in state A1 and the subsequent bit was 0, then the system was forced to transition to state C1. The interaction between the bit stream and the demon not only changed the state of the demon but also recorded the transition in the outgoing data. This served as the "memory" of the system.

The thermal reservoir and mass left all of that intact, but made some transitions more likely than others. The mass moved up when the demon transitioned from C to A, and down when the demon moved from A to C. Higher temperatures in the thermal reservoir fed more energy into the system, making the probability of raising the mass greater. Conversely, lower temperatures would fail to supply enough energy to raise the mass.

In other words, the transitions did not happen automatically: some were forbidden if the thermal fluctuations were too low. A cold reservoir might block clockwise motion, for example, while higher temperatures would tend to keep the mass in the higher position, blocking counterclockwise motion. In this way, thermal fluctuations—random and sometimes damaging to sensitive systems—could be converted into useful work, in this case raising the mass.

This, in turn, had an impact on the output bits. High temperatures made the demon act as an eraser: whatever the input data, once the block was raised to its maximum position, the demon would return a stream of all zeros. Under other circumstances, the demon would randomize the output stream.

So, the demon can convert random fluctuations into work, write its actions into the output stream, and/or erase the record of what it did. The recording and erasure steps are necessary to preserve the second law of thermodynamics, which says that the entropy of the entire system—demon, thermal reservoir, data stream, and mass—must increase, even as the demon lowers the entropy of the thermal reservoir by extracting usable energy from it.

While this work is purely theoretical as it stands, it's the first time someone has worked out the details of a quantum version of Maxwell's demon. The interesting aspects are as much in understanding quantum information theory as in possible future applications: it shows in principle how a purely mechanistic demon could convert random fluctuations to useable energy, and how a 19th century thought experiment could be made useful in the real world.